In Pasadena, Calif., the Christmases and New Years are seldom white, which is why one California Institute of Technology (Caltech) student thought it might be fun to add an unexpected snow flurry to the annual Rose Parade on New Year's Day. Wanting to ensure his impromptu dusting wouldn't scare anyone, the student first spoke with local police.

Instead of responding with a simple yea or nay, the police launched an investigation, recounts Thomas Mannion, assistant vice president for campus life at Caltech. Six different police departments and the Department of Homeland Security contacted the would-be prankster before authorities dropped the case.

As the US celebrates Halloween, a night of time-honored trickery, college campuses across the nation may find themselves the target of many a practical joke. What's changed is how these jokes are carried out. Cultural shifts have altered the boundaries of what's acceptable, and 9/11 has raised new security concerns. All of this has made administration-monitored pranking the norm for universities that wish to preserve the tradition.

For better or worse, the days of prank-first, question-later are gone. In an open letter to the Massachusetts Institute of Technology student body, which, like Caltech, has a longstanding history of pranks, Chancellor Phillip Clay wrote earlier this month, "We cannot deny the fact that what was tolerated in the past, and may even have been celebrated, is now viewed differently."

In the mid-'80s, for example, MIT students hacked the elevator system in a campus building. When passengers pushed a button, the car delivered them to a random floor. While the prank, or hack, as they're called at MIT, has attained legendary status, Kirk Kolenbrander, vice president for institute affairs, says that now such a stunt would likely make waves.

"That's clever, but at the same time our society today would say that there are real safety issues if that elevator is needed in an emergency," says Mr. Kolenbrander. "Our world has a different patience for those issues than it once did."

Even among the student body, tolerance for tomfoolery has begun to change. Following complaints in 2000 from several students at Harvey Mudd College in Claremont, Calif., about an annual prank where sophomores perform elaborate freshman room rearrangements – such as turning a dorm room into a campsite, complete with sod – administrators decided that rather than sacrifice their prank culture, they would refine it by creating a "no prank list."

"There is an implicit assumption that when you come to Harvey Mudd that you are willing to be party to pranks against you and your room," explains Guy Gerbick, associate dean of students. "We tell students during orientation, 'If you don't want to have certain things or yourself or any of your stuff pranked, let us know, and we'll put you on a list.' "

Over half the student body has registered. According to Mr. Gerbick, most make specific demands, such as not to interrupt sleep or meddle with a prized guitar or stuffed-animal collection. Only about 15 students have asked for no involvement whatsoever.

At both Harvey Mudd and Caltech, students must get administrative approval before they perform pranks – that way they can be left up for the entire campus to enjoy. When Mr. Mannion began working at Caltech 14 years ago, he was distressed by a decline in student pranks at the institution, which holds the No. 1 ranking on the all-time college prank list, according to the Museum of Hoaxes. Caltech took top honors for a 1961 Rose Bowl stunt, in which Washington students were tricked into proudly holding flip cards aloft to spell "CALTECH."

Hoping to create a climate more inviting to high jinks, Mannion now counsels students about potential pranks, and, if he gives the OK, campus police and janitors are not allowed to stop the stunt. Caltech even has a $10,000+ fund to finance student pranks.

For university police on campuses with an established pranking culture, officers "walk a fine line," says John DiFava, director of security and campus police services at MIT in Cambridge, Mass, In most cases, his department will not actively try to stop pranks, although if they see students trespassing, they will intervene.

"On one side of the equation you have a policy that says there are certain places with restricted access, and on the other side you have a tradition that's celebrated from all different quarters of the institute ... and we're caught in the middle," says Mr. DiFava. "It's a really tough position."

Despite any potential friction they can create, Mannion argues that good practical jokes serve an important role in higher learning. "Pranks are great for all kinds of things: organizational skills, social skills, publicity," he explains. Mannion wrote a letter of recommendation for a student applying to the Rhodes Scholar program largely based on abilities he demonstrated on a cross-country prank against MIT.

For Todd Gingrich, a Caltech senior who has flown all the way to Boston to prank MIT students, a good prank is an opportunity for students to demonstrate their technical skills in a creative manner. "It's a way to show that locking ourselves in our rooms and studying forever can actually lead to some practical and amusing results," he says.